JPS6039741B2 - High carbon low alloy steel with excellent toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high-carbon, low-alloy steel with excellent balling properties, which is used as bearings, working rolls for cold rolling, and the like.

Since bearings and working rolls for cold rolling are used under extremely severe abrasion conditions, they are particularly required to have excellent abrasion resistance.

As such materials, high carbon low alloy steels containing carbides in the maltesosite base and having high hardness have been widely used. However, although this material has excellent wear resistance, it frequently suffers from surface peeling, breakage, spalling, and other brittle fractures caused by microscopic cracks and defects, and its lifespan as a mechanical component is extremely short. There was a drawback. In order to prevent such brittle fracture accidents, it is first necessary to increase the impact value of the material and improve the balling properties. It has long been known that the impact value of a material is generally better as the grain size becomes smaller, and the toughness of the above-mentioned high-carbon, low-alloy steel can also be improved by refining the grains through heat treatment. Various attempts have been made to achieve this goal. However, despite such efforts, there are still dissatisfaction with the lifespan of mechanical parts such as bearings and cold rolling actuating rolls, and labor and cost still remain unsatisfactory due to the complexity of heat treatment operations required to refine crystal grains. The company was forced to bear an increased burden on employees, etc. The present invention overcomes the above-mentioned problems with bearings, cold rolling actuating rolls, etc., and satisfies both characteristics of wear resistance and ballability.
It is an object of the present invention to provide a new high-carbon, low-alloy steel that is also economically advantageous.

Brittle fracture accidents in bearings or rolling rolls occur starting from minute cracks caused by thermal shock, thermal fatigue, transfer fatigue, etc. As a result of detailed studies on various factors that are thought to affect the fracture toughness of low-alloy steel, we found that, contrary to the conventional general opinion, ``the larger the grain size, the lower the fracture toughness. We discovered the unique fact that "the crystal grain size of copper with a certain composition increases through heat treatment," and found that by increasing the grain size of copper with a certain composition, it is possible to improve fracture ballability and significantly improve the lifespan of mechanical parts. As a result, the present invention has been completed.

That is, the present invention is based on CO. 6~1.2%, S core 15~
1.6%, Mno. 15-1.6%, Nil. 0% or less, Cr2.6-6%, Mol. 0% or less, VI. The purpose of the present invention is to provide a high-wear, high-carbon, low-alloy steel with excellent ballability, which has a steel structure consisting of 0% or less residual iron and unavoidable impurities, and an average grain size of 30 m or more. The high-carbon low-alloy steel according to the present invention includes bearing materials that require wear resistance, working roll materials for cold rolling, etc. as main applications, and in order to meet the material characteristics for the application, chemical The component composition is defined as follows.

C is the most important element regarding the hardenability of steel, and is also an element useful for imparting wear resistance by combining with Cr and the like described below to form carbides.

If the amount added is less than 0.6%, a sufficient amount of carbide to provide good wear resistance cannot be obtained;
If it is added in excess of %, a uniform dispersion of carbides cannot be obtained, resulting in a decrease in rice grain quality. Therefore, it is preferably added in a range of 0.6 to 1.2%. Si and M
n is necessary as a deoxidizing agent in the steel manufacturing process.

Therefore, 0.15% or more of each is added. however,
Adding too much will impair the cleanliness of the resulting steel ingot.
In both cases, it is desirable that the upper limit is 1.6%. Ni is an element useful for improving hardenability.

However, if added in a large amount, it will inhibit the spheroidization of the carbide, making it impossible to obtain the required hardness and wear resistance. Therefore, it is preferably added at an upper limit of 1.0%. Cr is an important element that not only imparts toughness but also forms carbides* to increase hardness and wear resistance.

However, if the amount added is less than 2.6%, the effect will not be sufficient, while if it exceeds 6%, problems will arise in steel manufacturing technology, making it difficult to produce a sound steel ingot. For this reason, 2.6
It is preferable to add it in a range of % to 6.0%. Mo is
It is added as an effective element to improve competitiveness and increase hardness.

However, even if a large amount is added, no further improvement in hardenability can be expected and it is economically disadvantageous. In the present invention, addition of 1.0% or less is enough to achieve the purpose. V too,
Although it is a useful element for improving hardenability, it is expensive, so
Considering the economic effect, the upper limit of the amount added is 1.0%.

In addition, it is desirable that the amount of P and S mixed in as impurities be as small as possible, and in order to reduce the negative effect on toughness, preferably PO. 04% or less, SO. 0.03% or less.

Next, the relationship between crystal grain size and fracture ballability will be explained.
A, B, C and D having the chemical composition listed in Table 1
For steel, various grain sizes are given by heat treatment, and hardness H
Figures 1 and 2 show the fracture plowability values (k91 side - 3/2) of each steel when adjusted to v750. The crystals of each steel were adjusted by repeating the quenching treatment for steels A, B, and D, and by increasing the competition temperature for steel C. In addition, the hardness of each steel was adjusted by tempering. Table 1 Chemical composition of test steel (wt omitted) Curves A and B in Fig. 1 and curves C and D in Fig. 2 are the fracture susceptibility values of the above steels A, B, C, and D, respectively. shows.

From each figure, it can be seen that as the crystal grain size increases, the fracture ballability value improves, and in particular, it improves rapidly when the crystal grain size reaches 30 mm. Therefore, the crystal grain size is adjusted to 30 rm or more, preferably 50 m or more. In addition, in working rolls for cold rolling, if fine cracks of about 0.1 to 0.2 legs occur on the roll surface due to thermal shock during rolling,
Starting from this, brittle fracture accidents occur due to contact pressure during rolling, so it is desirable to have stability that can withstand fracture even if such cracks occur.

This stability can be evaluated by the level of the critical contact pressure, which is the critical point for brittle fracture occurrence, and as shown in the examples below, the stability is significantly superior to that of conventional materials. It has been confirmed that this is guaranteed. In addition, the crystal grain size referred to in the present invention can be defined by the average crystal grain size, and strictly speaking, the grain size of all crystals is 30 m.
It is not necessary that the particle diameter is above 30 mm, and particles having a particle size of less than 30 mm may be mixed. However, it goes without saying that it is desirable to eliminate the moat grain structure as much as possible and to have a uniform grain size. Next, the material properties of the steel of the present invention will be explained in more detail with reference to Examples. *Example 1 Steels 1 and 2 having the chemical compositions listed in Table 2 were used as test materials, and various grain sizes were given by heat treatment, and the respective fracture ballistic values and the results when cracks were present on the material surface were determined. The critical contact pressure at which brittle fracture occurs was measured.

In addition, in Table 2, sample material 1 was produced as a working roll material for cold rolling. Test material 2 is a high-carbon, low-alloy steel having the steel composition of the present invention, and is a comparative material equivalent to JISSK DII class used in cold dies that require wear resistance and especially plowability. Table 2 Chemical composition of sample materials (wt omitted)
Both sample materials 1 and 2 were previously subjected to spheroidizing and spheroidizing treatments, and the crystal grain size was adjusted and quenched by the heat treatment described below. m Heat treatment of sample material 1 and grain size: 'a) 850qox spot r・Air cooling → 60000×2 plate r・
Air cooling → 9300015 min/oil cooling, average particle size: 9 flea m
(b} 93000 x 3 hin/air cooling → 930003
Enhin/oil cooled, average particle size: 31 French m {c} 9300
0 x 3 heat/air cooling: repeated 5 times → 930°C x 3 heat/oil cooling, average particle weight: 130 French m (21 test villages 2
Heat treatment and grain flea: Standard quenching treatment (1050oo
×3㎇h; n・air cooling) Average grain law: loAm After each of the above heat treatments, the samples were returned to a maximum temperature of 200 qo to give various hardnesses, and then the fracture toughness values were measured.

In addition, the test samples 1 subjected to heat treatments a, b, and c are respectively referred to as material 1.a, material 1.b, and material 1.c. '3' Fracture toughness test results ``Relationship between hardness (Hv) and fracture toughness value (k9/side -3/2)'' of each sample material obtained from the same test
is shown in Figure 3.

In the figure, 'i} is 1・a material (average grain size: 9 mm, tii}
1.B material (31 m), 'III) 1.c material (130 Am), and [M] comparative steel, Sample Village 2 (10 m). Among the test materials 1, (material 1.a, material 1.b, material 1.c) having the steel composition specified by the present invention,
By making the crystal grain size approximately 30 m or more, as in 1-b material, it is given superior quality than 1-a material, which has undergone the conventional quenching process. At surface hardness Hv800 of the operating roll, the fracture toughness value improved by about 40% compared to the comparative steel S containing 12% Cr.
KDI12 indicates the same level. In particular, for the 1-c material with a crystal grain size of 130 fm, the fracture toughness value at Hv800 is approximately 83 kg Yanagi-3 No. 2, the highest value known to date for high-carbon, low-alloy steel. Compared to the above-mentioned 1.a material, which corresponds to the conventional hardened material, an improvement effect of 80% or more can be seen. {4} Critical contact surface pressure measurement results for brittle fracture occurrence Sample material 1 (1・a material, 1・b material, 1・
c material), the measurement results of the contact surface pressure limit for brittle fracture occurrence when a thermal shock crack on the 0.2 side occurs on the roll surface are shown in the third table.
Shown in the table.

Table 3 Limit contact surface pressure From Table 3 above, the larger the crystal grain size, the higher the contact surface that can withstand it. It is recognized that the above materials 1.b and 1.c allow approximately twice or more of the contact surface.

Example 2 Steels 3 to 10 having the chemical composition listed in Table 4 were used as test samples, and the average crystal grains were reduced to 35 to 45 m by heat treatment similar to the above-mentioned test materials 1 and 2. After adjusting the hardness to Hv7
A destructive strength test was carried out with the test materials set between 80 and 820.

Table 5 shows the results. Note that in Tables 4 and 5, Sample Materials 6, 7, and 10 are comparative materials. Table 4
Chemical composition of sample material (wt Tsutomu) Table 5 As is clear from Table 5, rice fracture properties, test material 6, which is a comparison material.
, 7 and 10, toughness values comparable to those of sample material 1 were obtained in all cases.

Regarding sample materials 6 and 7, the Ni amount exceeds 1%,
For this reason, it was found that the transformation point was lower and the amount of retained austenite was greater than that of other test materials. The presence of retained austenite is thought to be the reason why the rice grain quality decreased. Furthermore, it has been found that when Nj exceeds 1% and is subjected to sub-zero treatment after quenching, minute cracks occur due to transformation of retained austenite. This shows that Ni needs to be suppressed to 1% or less. Further, in one sample, the V content exceeded 1%, which caused a large amount of VC carbide to be formed, resulting in a decrease in fracture toughness. Therefore, it is desirable that V also not exceed 1%. As described above, the high-carbon, low-alloy steel with the composition and grain size according to the present invention has excellent rutting resistance, has an extremely low risk of brittle fracture accidents, and can be used as bearings, working rolls for spreading, etc. , product life can be greatly improved.

[Brief explanation of drawings]

Figures 1 and 2 are graphs showing the relationship between grain size and fracture toughness value of high carbon low alloy steel, and Figure 3 is a graph showing the relationship between hardness and fracture toughness value of high carbon low alloy steel. This is a graph showing. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1 C0.6-1.2%, Si0.15-1.6%, M
n0.15-1.6%, Ni1.0% or less, Cr2.6
~6.0%, Mo1.0% or less, V1.0% or less, the balance consists of iron and unavoidable impurities, and the average grain size is 30%.
A high-carbon, low-alloy steel with excellent toughness characterized by a toughness of μm or more.